New Sulfur Flow Battery for Affordable Long-Term Grid Storage

The “air-breathing” battery is energy-dense and made of abundant, low-cost materials

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A photo shows an “air-breathing” battery made of clear glass tubins with red caps and an orange liquid in one half.
Photo: Felice Frankel

The high cost of batteries still prevents them from being used to store renewable energy for times when the wind dies down or there’s no sun. Pumped hydroelectric storage is the cheapest known energy-storage technology today, but is limited by geography.

With a new battery, researchers at MIT say they have found the sweet spot for energy storage. The energy-dense battery could be the first to compete with the installed cost of pumped hydro and compressed-air storage, which cost around $100 per kilowatt-hour of energy stored. Scaled-up versions of the new battery could store electricity for a fifth of that, at $20/kWh. By comparison, Tesla claims its Gigafactory can produce batteries for around $125/kWh. 

The new battery might even have what it takes to replace fossil fuel “peaker” plants that can quickly inject power into the grid at high demand times. To compete with peaker plants, we need immense batteries that store energy from wind and solar for multiple days, even months, at an installed cost of around $50/kWh.

The device, reported in the journal Joule, is a type of flow battery, in which both the anode and cathode are liquid electrolytes. The anode in this case is sulfur dissolved in water, while the cathode is an aerated liquid salt solution that takes up and releases oxygen.

Lithium ions move between the electrolytes, and the salt solution at the cathode takes up or releases oxygen to balance the charge. During discharge, it takes up oxygen and the anode ejects electrons into an external circuit. When the oxygen is released, electrons go back to the anode, recharging the battery.

MIT materials scientist and engineer Yet-Ming Chiang says that his team’s main goal in building it was to keep costs to a minimum. They chose a water-based flow battery concept, which would have lower energy than a traditional battery but would be much cheaper per kWh.

After analyzing dozens of known battery materials, they chose sulfur because it’s cheap and abundant. Giant heaps of sulfur are produced as waste from tar sands refineries. “One existing stockpile of 4 million cubic meters, if turned into sulfur-based batteries, would store several times the entire pumped hydroelectric capacity in the world today (∼1.6 TWh),” the researchers write in an accompanying article in Joule.

The battery can store 20 to 40 Wh per liter of its electrolytes, making it 500 to 1,000 times denser than pumped hydro systems. Which means portable versions of this battery could be situated wherever they are needed near wind and solar farms. Plus, the cost of all the active materials in the battery is only $1/kWh, less than that for most any other rechargeable battery.

The battery is ideal for long-term storage because it is “scalable to a large size, made of earth-abundant materials, and has a stable chemistry in storage,” says Chiang. And as it gets bigger, storing energy gets cheaper. “System cost is a strong function of storage duration,” he says. “For long duration storage beyond a day, cost continues to drop and reaches $20 to $30 per kilowatt-hour.”

Editor’s note: This post was updated on 18 October to revise the energy density of the new battery from 20 to 40 kWh per liter to 20 to 40 Wh per liter.  

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